The present device generally provides focusing of divergent high-energy x-rays while maintaining good energy resolution, and more particularly relates to a device and method for bending a monochromator crystal with respect to two orthogonal axes to provide both horizontal and vertical focusing of an x-ray beam.
An x-ray produced at a light source will spread out or diverge as it travels from the light source. X-rays produced by a beamline with a 5 milliradian divergence, for example, will spread to 5 millimeters (mm) by the time they are 1 meter away from their source, and to 50 mm when 10 meters away. This is a problem for light source scientists, who want the highest possible x-ray flux on a small spot.
Previous technologies for x-ray focusing relied on mirror-like surface reflections to focus x-rays. These technologies demonstrated that x-rays can be focused by bending a Bragg crystal. This approach was the first which enabled the use of a synchrotron x-ray beam having a large horizontal divergence. In the years since, the technology has improved to minimize the anticlastic bending which degrades performance of this class of focusing monochromator, but such technologies still required large active surfaces as the x-ray energy increases and/or the grazing incident angle decreases. This requirement causes technical difficulties in error control and there are theoretical limitations on the divergence of the x-rays that can be focused. Moreover, serious theoretical and practical limitations remain, limiting such technologies to low x-ray energies and small x-ray divergence.
For X-rays with energies above 30 keV, the Bragg angle is small and it is difficult to implement traditional bending of the crystal. Because of the decreased Bragg angle, the beam's footprint on the crystal increases. Large crystals, of length approximately 100 mm, must be used, making the control of anticlastic bending difficult, if not impossible. For example, focusing of X-rays from 40 to 60 keV has been recently achieved by combining specialized bender, high-precision cutting of hinged crystals and higher index diffraction to increase the Bragg angle. Also, at high x-ray energies, the energy bandwidth of the monochromatic beam created is dominated by the vertical opening angle of the beam, which is of the order of a few tenths of a milliradian. The resulting energy resolution may be unacceptable for some applications. Finally, the bending radius required becomes extremely small at high x-ray energies, requiring extremely thin crystals, which is impractical for such long crystals.
The recent availability of powerful, third-generation high-energy synchrotron radiation sources, such as the APS in the United States, the ESRF in France, and Spring-8 in Japan, and the availability of superconducting wigglers have pushed the spectrum of x-rays to much higher energies than imaginable two decades ago. Thus, practical methods were needed to focus diverging high-energy x-rays so that these facilities would not be limited to using either lower energy x-rays or a tiny part of the large horizontal fan beam.
Commonly owned U.S. Pat. No. 7,508,912 to Zhong et al., the specification of which is incorporated herein by reference in its entirety for all purposes, discloses an x-ray focusing device utilizing a set of Laue crystals, named for German physicist Max von Laue, to diffract an x-ray beam, as opposed to reflecting the beam. Specifically, the invention described therein uses the lattice planes inside such crystals to monochromatize and focus the x-rays, thus allowing them to be almost perpendicular to the surface of the crystal. The transmission geometry renders the beam's illumination length small, reducing the control of the crystal's figure-error from a two-dimensional problem to a one-dimensional one. This new concept takes advantage of the fact that high-energy x-rays have enough penetrating power to go through the thickness of the Laue crystal.
As a result, the Laue geometry of the crystals provides advantageous anticlastic bending with reduced cost and ease of operation. Moreover, simple linear translation capabilities of the device disclosed in the '912 patent allowed for one-motion tuning of x-ray energy. Therefore, in addition to gains of focusing, an order-of-magnitude increase in the monochromatic intensity could be achieved while providing better energy resolution, compared to existing prior art Bragg crystals.
However, conventional x-ray focusing applications utilizing Laue crystals have thus far involved bending the focusing crystal in only one direction. Specifically, the crystalline structure of silicon (and other materials) selectively allows particular wavelengths of soft x-rays to be deflected at specific angles through the thickness of the crystalline material. Thus, when the crystal is bent laterally, focusing of the soft x-rays results.
In typical conventional x-ray focusing applications, monochromator crystals were generally purchased and/or machined flat. Where focusing in two planes (i.e., sagittal focusing) was desired, the crystals were bent laterally either using a four-bar fixture or by attaching fixed supports to two opposing ends of the crystal that would apply bending forces to the crystal through its rigid supports. Good focusing was therefore obtained in one plane, and due to the anticlastic shape that occurs naturally from lateral bending due to Poisson strain, some focusing in the meridional direction resulted. Those photons impacting the crystal from the radiation source that were not adequately focused in the meridional direction therefore made no contribution to the delivered photon brightness and were unfortunately discarded.
Moreover, while anticlastic curvature in the transverse direction (see “Spatially Resolved Poisson Strain and Anticlastic Curvature Measurements in Si Under Large Deflection Bending” by W. Yang, B. C. Larson, G. E. Ice, J. z. Tischler, J. D. Budai and K.-S. Chung of Oak Ridge National Laboratory, published in the Jun. 2, 2003 issue of Applied Physics Letters) results from inherent transverse shear forces and Poisson strain, this anticlastic curvature only contributes to meridional focusing of X-rays due to Poisson strain. This results in a saddle shaped crystal that inefficiently focuses photons. To date, no specific attempt has been made to control focusing in both the sagittal and meridional directions by changing the bending and therefore the three-dimensional shape of the crystal in two axes simultaneously.
Accordingly, it would be desirable to provide an x-ray focusing device and a method for bending at least one crystal in two orthogonal axes, sequentially or simultaneously, to control a monochromator crystal and develop an optimized shape in three dimensions. By focusing in both the sagittal and meridional directions, the bending of the crystal can provide added brightness and photon flux from the same radiation source in comparison with the resultant focusing in the meridional direction that occurs with the natural anticlastic shape from single axis lateral bending due to Poisson strain. Furthermore, since focal distances may be different for each application, the ability to fine-tune the focal length as needed for specific applications allows this invention to be used in many different applications.